This invention relates to a power piston for use in a booster and, more particularly, to a fastening mechanism for a valve body and a piston plate, when the power piston can be disassembled into the piston plate and the valve body.
A typical vacuum booster includes a valve body positioned in the center portion of a booster body or a casing and a piston plate positioned around the valve body and adapted to receive the differential pressure exerted on a diaphragm which divides the interior of the casing into two chambers. Usually, the valve body and the piston plate are fastened each other with bolts, while the inner circumferential portion of the diaphragm is clamped therebetween. The bolts are inserted through openings formed in the piston plate and then threaded into threaded holes provided in the valve body. Since the valve body is formed with air passages, the threaded holes are positioned radially outside the air passages, so that the outer diameter of the valve plate becomes considerably large. On the other hand, the threaded holes should have axial depth sufficient for achieving required tightening force, so that the thickness or the axial length of the valve body in its outer circumferential portion should necessarily be increased. The tightening stress and the thermal stress acting around the threaded hole portion present additional problems, particularly when the valve body is made of synthetic resin. In addition, since the heads of the fastening bolts project from the front surface of the piston plate, this imposes another problem in design and arrangement of the other members such as a fulcrum plate or the like and increases the axial length of the booster.